Unexamined Japanese Patent Publication No. Hei-9-237938 has disclosed the fact that a (111) face of metal nitride having a rock salt structure as an undercoat layer is used as a substrate in order to obtain Group III nitride compound semiconductor layers good in crystallinity. That is, in this official gazette, metal nitride having a rock salt structure is used as a substrate so that Group III nitride compound semiconductor layers are grown on a (111) face of the substrate.
Characteristic (such as stiffness, impact resistance, etc.) for retaining the function of a semiconductor device is required of the substrate of the device. It is conceived that the substrate needs a thickness, for example, of not smaller than 50 μm in order to retain the characteristic when the substrate is made of metal nitride.
Metal nitride having such a thickness, however, has been not provided as a raw material of an industrial product used for producing a semiconductor.
Therefore, an object of the invention is to be capable of forming a Group III nitride compound semiconductor layer having a good crystallographic structure by using an industrially easily available raw material. Accordingly, a semiconductor device according to the invention has a semiconductor layer of a good crystallographic structure and can be produced inexpensively.
From a different point of view, another object of the invention is to provide a Group III nitride compound semiconductor device having a novel substrate and a method for producing the same.